Field of the Invention
The present invention relates to a fuel cell system capable of purging the reaction gas that remains in the fuel cell body when the operation of the fuel cell is suspended. This invention also relates to the method for operating the fuel cell system.
As is well known, a fuel cell system takes the form of a laminated stack of single cells in which a matrix impregnated with an electrolyte is sandwiched between fuel electrodes and oxidizer electrodes which are immersed in a common electrolyte. Electric power is generated by supplying a fuel gas containing hydrogen and an oxidizer of air or oxygen to the stack. Depending upon the electrolyte used and the working temperature at which it is used, the fuel cell system is classified as a phosphoric acid type, an alkali type, a molten carbonate, or others too numerous to list.
When the operation of a fuel cell system is started after having been stopped, there is a danger of explosion because of the possibility that fuel gas rich in hydrogen is supplied with the air or oxygen remaining in the fuel system of the cell. Similarly, when the operation of a fuel cell system is stopped, the danger of an explosion is present. If fuel gas is left in the body of the cell, the pressure of the fuel gas will decrease because of the inner discharge of the fuel cell system or because of changes in temperature, and explosive gas will be formed by the air coming into the fuel side from outside the fuel system.
To solve these problems, efforts are being made to replace the gas. For example, when starting or stopping (including emergency stops) the operation of a fuel cell system, the gas in the fuel gas supply-exhaust system usually is replaced with inactive gas like nitrogen. The gas also is replaced in the fuel cell body.
When carrying out gas replacement in a conventional fuel cell system, inactive gas always is stored and controlled in pressure cylinders and storage tanks separate from the fuel/oxidizer supply system. The inactive gas is supplied from the storage tanks to the fuel cell reactive gas system whenever operation of fuel cell system starts or stops. However, this conventional practice requires, in addition to fuel control, troublesome control procedures These procedures include the constant monitoring of the quantity of inactive gas remaining in the inactive gas storage tank, securing inactive gas stock, and securing spare stock. Moreover, purchase and procurement, especially for movable power equipment which must have a large inactive gas storage tank, becomes large in size.
Accordingly, as disclosed in Japanese Patent Publication No. 58-32903, the applicant claimed a method to fill the oxidizing agent block chamber of a fuel cell system with nitrogen before stopping the operation of the fuel cell system. This is done by opening the external load circuit of the fuel cell system and, while supplying the fuel cell system with spontaneously diffusing air and fuel gas, connecting the external load circuit to an external short circuit installed in parallel to that external load circuit. This procedure discharges the fuel cell system and consumes oxygen present in the air of the oxidizing agent block chamber.
This method is effective for fuel cell systems using pure hydrogen without non-reactive components as fuel gas, or for fuel cell systems using fuel gas with only a few stacked cells, but has drawbacks if applied to fuel cell systems using a fuel gas with non-reactive components like a gas reformed from hydrocarbons (usually including CO.sub.2) or if applied to fuel cell systems with more cells. Under these circumstances, air and fuel gas are diffused and supplied to the fuel cell system in a volume equivalent to what has been consumed in the fuel cell system. Thus, the content of non-reactive components in the gas in the fuel gas block chamber, in particular, becomes higher as the electricity is discharged in the external short circuit. This causes the gas in the fuel gas block chamber to be denser than the fuel gas with rich hydrogen supplied through spontaneous diffusion from the exterior. Consequently, the difference between the densities of both gases causes the fuel gas with rich hydrogen from the exterior to be supplied only to the top of the stacked cells. Therefore, most of the gas components in the fuel gas block chamber of each of the stacked cells in the bottom are non-reactive CO.sub.2, and the discharge is stopped.
Moreover, the discharge of cells stacked in series also is stopped. Discharge therefore becomes impossible before oxygen in the air block chamber is completely consumed. This prevents the gas concentration to decrease, and hydrogen gas remains in the fuel gas block chamber on top of the cell stack. Replacement with inactive gas therefore is insufficient.